they do at least provide a convenient way to document the types and order of values to be passed, which at least would seem to me to help when someone is looking at a module written using them.

Please no, No, NO, NO, NO! Say it ain't so! Comments are a convenient way to document the types and order of subroutine parameters; prototypes most certainly are not. That is arguably true in just about any language. In Perl, it is doubly so because using prototypes in places where they aren't needed creates an enormous maintenance hassle.

I strongly encourage you to read Tom Christiansen's Prototypes in Perl which greenFox also mentioned in his reply above. You might be surprised to see how much trouble "prototypes" can get you into.